1. Field of the Invention
The present invention relates to a casting apparatus and method for producing ultra-large, thin-walled components and, more particularly, to a filtering molten metal injector system for producing ultra-large, thin-walled components that includes one or more filtering stages for filtering molten metal in the molten metal injector system.
2. Description of the Prior Art
The manufacturers of ground transportation vehicles, such as automobiles, support utility vehicles, light trucks, vans, buses, and larger capacity trucks have made major efforts in recent years to reduce vehicle weight. Weight reductions increase fuel efficiency and reduce harmful atmospheric emissions of ground transportation vehicles. Presently, a majority of the body components for ground transportation vehicles are formed from individual steel components that are assembled via resistance spot welding. For example, the floor pan frame of an automobile is normally constructed from a number of individual steel stampings that are spot welded together. It would be advantageous to produce such body components as a single, ultra-large casting. As a result, the costs associated with producing and assembling multiple steel stampings would be eliminated. The same technology would also be suitable for components in the aerospace industry.
There are several known methods for producing thin-walled castings. Examples include: high-pressure cold chamber vacuum die casting, premium sand casting, a level pour process practiced by Alcoa Inc. for producing components for the aerospace industry, and low-pressure hot chamber injection. Low-pressure hot chamber injection is particularly well-suited for producing components made from nonferrous metals having a low melting point, such as aluminum, brass, bronze, magnesium, and zinc.
Typical casting arrangements known in the prior art utilize a reciprocating piston located within a cylinder for injecting molten metal into a casting die. For example, U.S. Pat. No. 4,991,641 to Kidd et al. discloses an apparatus that includes a supply tank containing molten metal and a cylinder located in the supply tank having at its base a connection to an injection passageway, which leads through the tank to a casting die located outside the tank. A reciprocating piston is located in the cylinder for injecting molten metal into the injection passageway leading to the casting die. The injecting or pumping stroke of the piston is directed toward the bottom of the supply tank, or during the xe2x80x9cdownstrokexe2x80x9d of the piston. Other prior art casting devices are disclosed in U.S. Pat. No. 5,082,045 to Lambert; U.S. Pat. No. 5,181,551 to Kidd et al.; and U.S. Pat. No. 5,657,812 to Walter et al.
The piston arrangement disclosed, for example, by the Kidd patent, which pumps molten metal during the downstroke of the piston, has a tendency to disturb the metal oxide film surface of the molten metal contained in the supply tank. Consequently, undesirable metal oxides are often pulled into the cylinder from the metal oxide film surface, or formed in the cylinder due to the action of the downward directed piston. These metal oxides are then injected into the casting die along with the molten metal, which results in an inferior cast product. Further, these metal oxides are typically large particles that can score and damage the internal surfaces and seals of the piston-cylinder arrangement, as well as score and damage the injection passageway leading to the casting die. In addition to metal oxide formation, piston arrangements in which the pumping stroke is directed downward in a supply tank of molten metal are known to pull air into the piston cylinder, which forms air pockets in the cylinder. These air pockets, or air bubbles, are injected into the casting die along with the molten metal, which forms occlusions within the cast product. A poor quality final product generally results.
Accordingly, it is an object of the present invention to provide a molten metal injector system for casting of inexpensive, but high-quality thin-walled components of such size and complexity that traditional stamping assemblies made from multiple components could be replaced with a single, ultra-large, thin-walled component. It is another object of the present invention to provide a filtering molten metal injector system and method for reducing or eliminating the introduction of undesirable metal oxides into a casting die used for producing the ultra-large, thin-walled components.
The above objects are accomplished with a filtering molten metal injector system and method according to the present invention. The present invention combines the advantages of low-pressure, hot chamber molten metal injection with a filtering molten metal injector, which may include multiple molten metal filters for filtering molten metal before injection into a casting mold. The molten metal injector of the present invention includes a holder furnace for containing a supply of molten metal. A casting mold is supported above the holder furnace and has a bottom side facing the holder furnace. The casting mold defines a mold cavity for receiving the molten metal from the holder furnace.
A molten metal injector is supported from the bottom side of the mold and projects into the holder furnace. The molten metal injector includes a cylinder defining a piston cavity housing a reciprocating piston for pumping the molten metal upward from the holder furnace to the mold cavity. The cylinder and piston are at least partially submerged in the molten metal when the holder furnace contains the molten metal. The cylinder or piston includes a molten metal intake for receiving the molten metal into the piston cavity when the holder furnace contains the molten metal. A conduit connects the piston cavity to the mold cavity. A first molten metal filter is located in the conduit for filtering the molten metal passing through the conduit during the reciprocating movement of the piston.
The molten metal intake may be a valve connected to the cylinder for providing fluid communication between the piston cavity and the molten metal in the holder furnace when the holder furnace contains the molten metal. The valve may be configured to open for inflow of the molten metal during a downstroke of the piston away from the bottom side of the mold, and configured to close during a return stroke of the piston toward the bottom side of the mold. A second molten metal filter may be used to cover the inlet to the valve for filtering the molten metal flowing into the piston through the valve during operation of the molten metal injector.
The cylinder of the injector may define an open end opposite the piston. The molten metal intake may be a gap formed between the piston and the open end of the cylinder during the reciprocating movement of the piston. The second molten metal filter may enclose the open end of the cylinder for filtering the molten metal flowing into the piston cavity through the gap during operation of the molten metal injector.
The molten metal intake may further be an aperture defined in a sidewall of the cylinder. The aperture may be open for inflow of the molten metal into the piston cavity during the reciprocating movement of the piston. The second molten metal filter may be used to cover the aperture for filtering the molten metal flowing into the piston cavity through the aperture during operation of the molten metal injector.
Furthermore, the molten metal intake may be a ball check valve incorporated into the piston for providing fluid communication between the piston cavity and the molten metal in the holder furnace when the holder furnace contains the molten metal. The ball check valve may be configured to open for inflow of the molten metal during a downstroke of the piston away from the bottom side of the mold, and configured to close during a return stoke of the piston toward the bottom side of the mold. The second molten metal filter may be used to cover the inlet to the ball check valve for filtering the molten metal flowing into the piston cavity through the ball check valve during operation of the molten metal injector.
The present invention is also a method of filtering molten metal in a molten metal injector for use with a casting mold having a mold cavity. The method preferably includes the steps of: providing a supply of molten metal; providing the molten metal injector, with the molten metal injector including a cylinder defining a piston cavity housing a reciprocating piston, and with at least one of the cylinder and piston including a molten metal intake for receiving molten metal from the supply of molten metal into the piston cavity; connecting the piston cavity to the mold cavity via a conduit; placing the molten metal injector in fluid communication with the supply of molten metal such that the cylinder and piston are at least partially submerged in the supply of molten metal; receiving the molten metal from the supply of molten metal into the piston cavity via the molten metal intake; pumping the molten metal from the piston cavity to the mold cavity through the conduit with the piston; and filtering the molten metal in the conduit before the molten metal enters the mold cavity.
In addition, the method may include the steps of: filtering the molten metal during the step of receiving the molten metal from the supply of molten metal into the piston cavity via the molten metal intake; and filtering the molten metal returning to the piston cavity through the conduit after the step of pumping the molten metal from the piston cavity to the mold cavity through the conduit with the piston.
Further, the method may include the steps of: placing a first molten metal filter in the conduit such that the step of filtering the molten metal in the conduit before the molten metal enters the mold cavity is performed by the first molten metal filter; covering the molten metal intake with a second molten metal filter; and filtering the molten metal with the second molten metal filter during the step of receiving the molten metal from the supply of molten metal into the piston cavity via the molten metal intake.
Further details and advantages of the present invention will become apparent from the following detailed description, in conjunction with the drawings, wherein like parts are designated with like reference numerals.